LINQ String comparison with NULL handling

Question

I am using a LINQ expression like this

Attention attention = debtor_response.DebtorEntry
                .Address.AttentionList.Where(p => p.JobTitle.ToLower() == "valuetocheck")
                .FirstOrDefault();

This indeed worked in normal cases. But under some cases it returns an exception

 Value cannot be null.\r\nParameter name: source

The possible reasons in my thoughts are

JobTitle might be null in some cases

So how can i handle this correctly in the above LINQ and get rid of the exception

score 6 · Accepted Answer · 2018-06-15 04:02:47Z

I suspect AttentionList is null, because the method signature for Enumerable.Where is:

public static IEnumerable<TSource> Where<TSource>(
    this IEnumerable<TSource> source,               <-------
    Func<TSource, bool> predicate
)

Your error said that a parameter named source is null, which matches what this extension method throws:

Exception: ArgumentNullException
Condition: source or predicate is null.

Try changing your code to the following:

Attention attention = debtor_response.DebtorEntry
    .Address
    .AttentionList?.Where(p => p.JobTitle.ToLower() == "valuetocheck")
               // ↑
    .FirstOrDefault();

Notice the added ? on line 3? If AttentionList is null, the Null-conditional Operator will avoid calling Where

How did I come to this conclusion

You're getting an ArgumentNullException, but you only have three function calls:

Enumerable.Where          (extension)
Enumerable.FirstOrDefault (extension)
String.ToLower

ToLower doesn't have any parameters, so it can be ruled out. And if JobTitle were null, you would get a NullReferenceException.

Both extension methods have a parameter called source. However, Where is called first and can't return a null value to FirstOrDefault. So the culprit must be Enumerable.Where. It's source parameter is null, and that parameter is AttentionList.

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